NGC 7009

THE SATURN NEBULA

In Aquarius

See the spectrum.

From Jim Kaler's STARS; Return to Planetary Nebulae

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"Of wonderfully intricate structure" (Curtis), the planetary nebula NGC 7009, the "first planetary," displays a pair of jug-handle-like "ansae" that give it the name "The Saturn Nebula." On the left is a drawing made from a series of photographs, the gaseous nebula perhaps appearing as its discoverer William Herschel may have seen it.

In Herschel's words about his discovery object (from an article in the Philosophical Transactions of the Royal Society in 1785): "I shall conclude this paper with an account of a few heavenly bodies, that from their singular appearance leave me almost in doubt of where to class them. The first precedes Nu Aquarii 5'.4 minutes in time [5.4 minutes, 1.25 degrees], and is I' (minute of arc) more north ... The planetary appearance of the first two is so remarkable, that we can hardly suppose them to be nebulae; their light is so uniform, as well as vivid, the diameters so small and well defined, as to make it almost improbable that they should belong to that species of bodies." Thus was born the oft-confusing term, by which Herschel merely meant "disk-like." After all, who had a better right than the discoverer of the first planet since ancient times? (While Lyra's Ring Nebula was already known, it was not placed into the category of planetary nebulae until later.) Herschel then went on to discover (in NGC 6543) that planetary nebulae have central stars

On the right is a Hubble Space Telescope image made nearly a century later, showing the vast improvement in astronomical imagery as well as the immense complexity of the nebula. The hot (90,000 Kelvin), blue, 13th magnitude star at the center is the old nuclear-burning core of what was once an extended giant star, while the surrounding nebula is the inner part of the star's lost envelope that has been structured by the remaining star's hot wind. The distance is not well known. If at 2000 light years, the nebula is 3/4 of a light year long. The central star seems faint to us only because of its distance and because the vast majority of its radiation comes out as energetic ultraviolet light. In reality, the star thousands of times more luminous than the Sun. Still heating at constant luminosity, the star will eventually cool and dim to become one of the many white dwarfs that dot the cosmos.

The ansae, called "FLIERS," are high-speed jets shot out of the central star, probably along the star's rotation axis. While NGC 7009's are the most prominent, they are seen in a number of nebulae, notably NGC 3242, NGC 6543, and NGC 2371- 2. Their origins and significance are not understood.

Left: Image and quote by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: B. Balick, J. Alexander (U. Wash.), A. Hajian (USNO), Y. Terzian (Cornell U.), M. Perinotto (U. Florence), P. Patriarchy (Arcetry Obs.), and NASA.

THE AMAZING SPECTRUM OF NGC 7009

The seven panels below show the extraordinarily rich blue-to- ultraviolet emission spectrum of NGC 7009, covering from about 4800 Angstroms to the atmospheric cutoff near 3100 Angstroms. The Angstrom, a hundredth of a millionth of centimeter, is a standard unit of wavelength. "Forbidden lines," produced in heavier ions by collisions with electrons freed mostly from hydrogen, are indicated by square brackets. The term comes from their being forbidden according to simplified atomic theory. They are weak or absent in the lab, but under low-density nebular conditions can achieve great strength. They were first seen in 1864 in the spectrum of NGC 6543 by Sir William Huggins, but remained unidentified until 1928 when I. S. Bowen figured them out. The lines without brackets are permitted "recombination lines" caused by recapture of electrons by ions. The electrons are commonly caught in higher allowed orbits and then skip to lower energies, radiating their energies at specific wavelengths as they go. See the spectra of IC 418, the Ring Nebula, and the Orion Nebula for additional discussion.

This historical slit spectrogram (broken into seven overlapping parts) was taken in 1961 by Lawrence Aller using the coude spectrograph of the 100-inch telescope at the Mt. Wilson Observatory. The weakest lines have strengths nearly a ten- thousandth that of the standard H-Beta line at 4861 Angstroms (which is not shown). Note the remarkable ionized oxygen (O II) spectrum, the confluence of the Balmer lines in the fifth panel, which are visible to wavelengths shorter than that of H30, and the Balmer continuum that runs to wavelengths shorter than the "Balmer limit" at 3646 Angstroms. (The Balmer lines and continuum are the set of electron transitions from higher orbits to orbit 2.) The lines marked "L.A." are from mercury vapor street lights in Los Angeles. Vastly more lines lie at both longer and shorter wavelengths; this spectrum is just a sample.

NGC 7009
Panel 1. See full resolution. The second spectrum line from the right is a flaw. The iconic 4686 line of ionized helium (He II) to the right of center is very strong, as are the forbidden triply-ionized [Ar IV] lines.

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Panel 2. See full resolution . The spectrum is dominated by the H-Gamma line at 4340 Angstroms toward the left. The [O III] line from doubly ionized oxygen at 4363 Angstroms is used along with those at 4959 and 5007 Angstroms (the "nebulium" lines: see IC 418 and the Ring Nebula) to measure nebular temperatures. Just to the left of it at 4358 Angstroms is a line from Los Angeles street lights (this and others giving mercury vapor lamps their blue color.) The strong neutral helium (He I) line at 4471 Angstroms to the right, along with the 4686 Angstrom He II line seen in Panel 1, is used to measure nebular helium compositions.

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Panel 3. See full resolution. The spectrum is dominated by the H-Delta line at 4101 Angstroms toward far left. To the right, at a wavelength of 4267 Angstroms, is an important line of ionized carbon (C II) used to get nebular carbon compositions.

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Panel 4. See full resolution. Three more hydrogen lines, H-Epsilon (or H7) at 3970 Angstroms, H8 at 3889, and H9 at 3835 show the hydrogen spectrum beginning to pile up toward the limit at 3646 Angstroms as seen in the next panel. The forbidden doubly ionized neon [Ne III] line to the left of center at 3868 Angstroms is crucial for obtaining neon compositions. It is coupled to the 3967 line at center that is usually blended with the hydrogen and helium lines to either side of it. Note the highly ionized iron line at 3895 Angstroms.

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Panel 5. See full resolution. We arrive at the dramatic confluence of the Balmer lines and the beginning of the Balmer continuum, which stretches out to the left. It's marred somewhat by a couple strong lines from Los Angeles. The critical forbidden ionized oxygen lines at 3726 and 3728, used to derive nebular densities (and together nebular oxygen compositions), are well separated. At lower resolution they are often blended into one.

NGC 7009
Panel 6. See full resolution. The Balmer continuum weakens toward shorter wavelengths; note the strong O III lines from doubly-ionized oxygen. They are pumped up to higher intensities by a fluorescent mechanism. Numerous neutral helium lines, which are cascading toward their own limit, fall to the right of center.

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Panel 7. See full resolution. The 3203 line of He II, coupled to the 4686 He II line in Panel 1, is also strong as is the last of the observed O III lines. Beyond this point little radiation penetrates the Earth's atmosphere, compliments mostly of ozone.

From L. H. Aller and J. B. Kaler, Astrophysical Journal, vol. 139, p. 1074, 1964.